Polymers such as amide-imide polymers, imide polymers, sulfone polymers, fluoro polymers, and olefinic polymers are known as materials for constituting porous films. Porous films comprising such materials are produced, for example, by a method (phase conversion method) in which mixtures containing the polymers are cast as films and then introduced to solidifying liquids. The resulting films produced by this method using the polymers, however, have skin layers (dense layers), and they have substantially no open pore or, if any, have low rates of open pores on their surfaces. For example, there are disclosed porous films comprising a polymide as a porous film using an imide polymer as a material, and production methods thereof (e.g., Japanese Unexamined Japanese Patent Application Publication (JP-A) No. 2001-67643, JP-A No. 2001-145826, and JP-A No. 2000-319442). These porous films must be produced via solvent substitution rate adjustors so as to open holes (pores) on their surfaces, thereby require complicated production processes, and still have insufficient rates of open pores and permeability.
When porous films are used in such forms as to be exposed to chemicals such as highly polar solvents, alkalis, and acids, the porous films may be dissolved and/or deformed as a result of swelling. As a possible solution to these problems, the chemical properties of the films may be improved by changing chemical substances constituting the porous films. However, there are a variety of requirements for such porous films on, for example, heat resistance, flexibility, hardness, color, easiness in production, pore size, porosity, porous structure, price, strength, and chemical properties, and it is difficult to choose constitutive materials that satisfy all of these requirements in good balance.